The present invention relates to an air-fuel ratio control system for an engine mounted on a vehicle and having a learning control function.
An electronic control fuel injection system generally determines an injection quantity T.sub.i by compensating a basic fuel injection quantity T.sub.p by various compensation factors.
The basic quantity T.sub.p is the injection quantity to obtain a theoretical air-fuel ratio and is calculated by the following equation (1) with a suction air quantity Q and an engine speed S.sub.E : EQU T.sub.p =K.times.Q/S.sub.E ( 1)
where K is a constant.
The actual fuel injection quantity T.sub.i is set by multiplying the basic quantity T.sub.p by various correction coefficients corresponding to various operational conditions of the engine. The various correction coefficients include various increase correction coefficient COEF for adapting the air-fuel ratio to that of the operational condition by adding an acceleration correction coefficient, an air-fuel ratio feedback correction coefficient .alpha. for the theoretical air-fuel ratio, and a voltage correction coefficient T.sub.s. The air-fuel ratio is controlled by the actual fuel injection quantity T.sub.i according to the following equation (2): EQU T.sub.i =T.sub.p .times..alpha.COEF+T.sub.s ( 2)
In Order to keep the air-fuel ratio to the theoretical ratio, an air-fuel ratio sensor such as an oxygen sensor exposed in an exhaust pipe measures oxygen density of exhaust gases and a controller calculates an actual air-fuel ratio of the induced mixture. Air-fuel ratio feedback control is performed by the correction coefficient .alpha. in dependency on a difference between the calculated air-fuel ratio and the theoretical air-fuel ratio.
However, the air-fuel ratio feedback control requires a long time to converge the actual air-fuel ratio to a reference air-fuel ratio if the deviation between the reference ratio and the actual ratio is large. Furthermore, it is possible for the control of the air-fuel ratio to be disabled by instabilities such as overshoot or hunting of the air-fuel ratio when an engine operating condition rapidly changes or when the actual fuel injection quantity misses a control output in dependency on factors changing with the lapse of time.
Accordingly, more precise air-fuel control is realized by learning control having a learning value calculated by the difference between the actual air-fuel ratio and the reference ratio in order to improve the convergency of the feedback control, to compensate for deteriorations of individual parts or differences between the characteristics of each part, and to precisely correct the air-fuel ratio within the region in which air-fuel ratio feedback control cannot be performed. Namely, if a learning correction coefficient denotes K.sub.BLRC, the fuel injection quantity T.sub.i is calculated by the following equation (3): EQU T.sub.i =T.sub.p .times..alpha..times.COEF.times.K.sub.BLRC +T.sub.s( 3)
and the air-fuel ratio is controlled by the fuel injection quantity T.sub.i corrected by learning.
Such air-fuel ratio control by learning is disclosed in Japanese Patent Laid-Open No. 61-72843 (1986). In the prior art, a plurality of learning values are respectively set corresponding to engine load. Each value has a common learning term commonly included in all operational regions of the engine, and some individual learning terms each corresponding to the operational region. After the values of the individual learning terms are respectively corrected in accordance with the air-fuel ratio feedback correction coefficient .alpha., the deviation is calculated between an average value of all individual learning terms and a reference value. Next, mutual correction is performed by subtracting the deviation from each individual learning term and by adding the deviation to the common learning term. In the technology shown in the prior art, a corrective range of the common learning term is set broader than a corrective range of the individual learning term.
Now, a cause influencing an air-fuel ratio, mainly includes two factors of a suction air quantity measurement system and a fuel injection system. In the measurement system, the actual air-fuel ratio deviates from the reference air-fuel ratio because of the deterioration of an intake air quantity sensor and the like, while the actual one deviates from the reference because of the deterioration of an injector, pressure regulator, and the like, in the injection system. Both deteriorations, of the measurement system and the injection system have different characteristics as shown in FIG. 9. Namely, the deviation of the air-fuel ratio by the deterioration of the injection system changes substantially in all alike according to the change of the intake air quantity Q. On the contrary, the deviation by the deterioration of the measurement system increases according to the increment of the intake air quantity Q. In the region of the intake air over the predetermined value, the deviated amount by the deterioration of the measurement system is greater than the amount by the injection system. Accordingly, the discrepancy of the detected intake air quantity to the actual quantity, which is caused by the deterioration of the intake air quantity sensor, is different from the discrepancy of the calculated fuel injection quantity to the actual injection quantity which is caused by the deterioration of the injector, pressure regulator because of the difference of the operational range and the deterioration characteristics. Therefore, in the learning control, the learning values vary in response to the change of the intake air quantities. As a result, it is problem that the controllability is deteriorated by setting the learning value by only single parameter such as the engine load.
On the other hand, the technology for performing the learning control by using two parameters, is disclosed in Japanese Patent Laid-Open No. 60-93150 (1985).
In the prior art, an air-fuel ratio is corrected not only during the air-fuel ratio feedback control but also in the region where the air-fuel ratio feedback control is not performed. A learning correction coefficient is stored in a three-dimensional map on a random access memory (RAM) in dependency on an operational condition of the engine such as the engine speed and the engine load. The air-fuel ratio is controlled by correcting the constant K in the equation (1) to calculate the basic fuel injection quantity T.sub.p. The correction is achieved on the basis of the difference between the learning correction coefficient and an initial value only when predetermined number of the coefficients in the RAM are renewed over the predetermined times, and have the differences against the initial value in the same direction, respectively.
However, the map storing the learning correction coefficients requires a large memory capacity. Since the map has many regions which are not performed the learning, it is necessary to correct the learning value of the regions by presumption. Furthermore, since the fuel injection quantity is calculated by using the learning value corrected by the presumption, it is problem to lack precision at controlling the air-fuel ratio.
Still furthermore, as the above prior art has the construction that both deteriorations of the measurement and injection systems are learned together and stored in one map of the memory corresponding to the engine speed and the engine load, the prior art has a problem that it is impossible to individually detect each degree of the deteriorations of the measurement system and the injection system. Accordingly, it is impossible to correct the basic fuel injection quantity which is only influenced of deterioration of the measurement system by the aforementioned learning value, so that ignition timing control or the like using the basic fuel injection quantity as a controlling parameter receives a bad influence in the control precision.
On the other hand, in a vehicle having a canister purge system which adheres a vaporized fuel in a fuel tank to a canister for a time and returns the fuel to the engine during driving, the learning value changes corresponding to the change of the air-fuel ratio in dependency on the change of the purge quantity of the vaporized fuel. Therefore, the discrepancy of the learning value in each operational region reduces the control accuracy. Driving characteristics and exhaust emission of the vehicle deteriorate by the air-fuel ratio being too lean after the purge ends because it takes a long time to return the learning value to the value before the purge starts in accordance with the disappearance of the vaporized fuel adhered with the canister. Regarding this problem, applicants of this application disclose the technology of learning control by selectively using a learning table during the canister purge and a learning table at the time not to perform the purge, which is shown in Japanese Patent Laid-Open No. 61-1127 (1986), but there has not been disclosed a learning control in dependency on the difference of the deterioration characteristics between the measurement and the injection systems.